General Controlled Sulfidation toward Achieving Novel Nanosheet‐Built Porous Square‐FeCo₂S₄‐Tube Arrays for High‐Performance Asymmetric All‐Solid‐State Pseudocapacitors

Abstract

A significant advance toward the design and fabrication of a novel hierarchical supercapacitor electrode consisting of FeCo2S4‐tubes with well‐defined square cross‐section and intersecting nanosheets built porous shells on a 3D porous Ni backbone via controlled sulfidation is reported. This general method allows template‐free synthesis of metal sulfides tubular structures with polygonal cross‐sections and also fine control over the nanostructure leading to both maximized porosity and saturation sulfidation. New insights into concentration and time dependent sulfidation reaction kinetics are proposed. The FeCo2S4 electrode achieves a specific capacitance reaching 2411 F g‐1 at 5 mA cm‐2 and good rate capability, which are superior over those for nanotube arrays of other ternary transition metal sulfides. This is attributed to rich redox reactions, the highly porous but robust architecture as well as high electrical conductivity. Especially such porous shells effectively avoid “dead volume”, thus improve the utilization ratio of the electrode material. Asymmetric solid‐state device applying the FeCo2S4 as positive electrode and N‐doped graphene hydrogel film as negative electrode has a high cell voltage of 1.6 V and thus delivers considerably higher energy density of 76.1 W h kg‐1 (at 755 W kg‐1) than those reported for similar devices.

Iron‐Cobalt Bi‐Metallic Sulfide Nanowires on Ni Foam for Applications in High‐Performance Supercapacitors

“…Furthermore, the OH − ions play an important role in the electrochemical oxidation and reduction of cobalt sulfides as reported previously . Meng et al . and Deng et al .…”

Fabrication of Porous Mesh‐Like FeCo2S4 Nanosheet Arrays on Ni Foam for High Performance all Solid‐State Supercapacitors and Water Splitting

“…The Fe 2p and Co 2p have two shake‐up satellites ,. As described in Figure B, the binding energies at 709.3 eV and 721.9 eV, the other at 712.5 eV and 716.6 eV of the Fe 2p 2/3 and Fe 2p 1/2 peaks are assigned to Fe 2+ and Fe 3+ , respectively ,. The weak satellite peaks indicate that the Fe species are mainly existed in the form of Fe 3+ .…”

“…The Gaussian fitting method was used to obtain the typical fitted Fe 2p, Co 2p and S 2p XPS spectra (Figure B−D). The Fe 2p and Co 2p have two shake‐up satellites ,. As described in Figure B, the binding energies at 709.3 eV and 721.9 eV, the other at 712.5 eV and 716.6 eV of the Fe 2p 2/3 and Fe 2p 1/2 peaks are assigned to Fe 2+ and Fe 3+ , respectively ,.…”

“…The observed redox peaks can be attributed to reversible Faradaic redox process of Co 2+ /Co 3+ /Co 4+ and Fe 2+ /Fe 3+ . The electrochemical reactions between the sulpho‐spinel materials and electrolyte ions are proposed as follows: …”

“…In the blue curve, the peaks at 30.85°, 36.53°, 44.26°, 58.67° and 64.52° correspond to the (220), (311), (400), (511) and (440) diffraction, confirming the formation of FeCo 2 O 4 spinel phase (Fd3 m space group) . In the red phase, the peaks of Fe−Co sulfide are consistent with the XRD peaks of FeCo 2 S 4 in previous reports, although there is no standard PDF card ,. The FTIR spectrum images of the powered Fe−Co precursor, oxide and sulfide nanosheets are shown in Figure B.…”